1. Field of the Invention
This invention relates to an apparatus and method for implementing a capacitance pressure transducer comprising two fused quartz cylinders that are connected together at respective first ends and concentrically aligned relative to one another so that the capacitance of the transducer can be determined by the size of the gap which is established between the cylinders. The present pressure transducer has a particular application to be lowered into an oil or gas well for accurately sensing subsurface pressure conditions therein.
2. Prior Art
Capacitance pressure transducers comprising a pair of fused quartz cylinders that are concentrically aligned relative to one another so that a gap is formed therebetween are known to the prior art. However, the cylinders of such transducers are connected together at each of the first and opposite ends thereof. The conventional technique by which to interconnect the cylinders commonly includes a step of direct fusion or welding. Characteristic of such a direct fusion technique is the relatively high fusing temperatures (e.g. approximately 1600 degrees C.) required for melting fused quartz and for forming a fused quartz-to-fused quartz bond between the cylinders. However, such characteristic high temperatures are deleterious to a suitable fabrication of a fused quartz capacitance pressure transducer.
More particularly, the cylinders and especially the inner cylinder, have been known to crack at both ends thereof during the cooling phase of the pressure transducer fabrication technique. The likeihood of cracking is increased as a consequence of any asymmetrical alignment of the cylinders at opposite ends of the transducer. An attempt to eliminate the undesirable cracking has included thickening both ends of the transducer. However, such attempt has proven to be both time consuming and relatively costly.
Moreover, because of the high thermal conductivity of quartz, the (e.g. metal film) electrodes thereof have been known to either experience a change in characteristics or peel away from the cylinder surfaces to which they are applied during exposure to the high fusing temperatures. What is more, the high fusing temperatures sometimes cause an undesirable distortion in the configuration of the inner and/or outer cylinder, because quartz has a tendency to become plastic at temperatures in the order of 1600 degrees C. Any non-concentricity in the inner cylinder is reflected as a shear or tensile stress thereon when the outer cylinder is pressurized. Inasmuch as quartz can only tolerate relatively low shear and tensile stresses, the inner cylinder may crack during pressurization. The likelihood of non-concentricity is amplified when the inner and outer cylinders are connected together at both ends, as is common to the geometry of conventional capacitance pressure transducers.
Examples of capacitance pressure transducers which are illustrative of the foregoing conventional geometry and fabrication techniques may be found while referring to one or more of the following United States patents:
U.S. Pat. No. 3,750,476, Aug. 7, 1983. PA1 U.S. Pat. No. 4,064,549, Dec. 20, 1977.
Metal hydride processes have previously been known to those skilled in the art. For example, the article entitled "Active-Metal Soldering of Crystalline Quartz", M. E. Knoll, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 32, No. 1, January, 1961, relates to a technique for applying electrodes to quartz crystal resonators. Moreover, a metal hydride technique is sometimes used in the preparation of ceramic-to-metal seals on vacuum tubes and vacuum apparatus. For a generalized discussion of a metal (e.g. titanium) hydride technique as described above, reference may be made to VACUUM SEALING TECHNIQUES, A. Roth, Pergamon Press, Ltd., 1966 at page 211. However, the application of a metal hydride technique has been limited to the fusing of metal to a ceramic. Nothing is known in the prior art which relates to a process for interconnecting two pieces of fused quartz or glass by means of a metal hydride solder process, as is described and claimed hereinafter.